The invention relates to an apparatus for heat treating fine-grain solids, particularly cement raw material, comprising a rotary tubular kiln, a multi-stage cyclone preheater through which the waste gases of the rotary tubular kiln pass, and a bypass conduit connected to the conduit between the rotary tubular kiln and the cyclone preheater, for branching off part of the kiln waste gases, a mixing chamber with a fresh air connection being provided in said bypass conduit for cooling the branched-off sub-flow.
In the pretreatment of fine-grain solids, in particular cement raw material, in plants of the aforementioned type the waste gases of the rotary tubular kiln frequently contain noxious materials (such as alkalis and chlorides) which can produce a troublesome accumulation in the preheater. To avoid such accumulation, it is known to withdraw part of the kiln waste gases via a so-called bypass conduit (bypassing the cyclone preheater), to cool them with the aid of fresh air in a mixing chamber and then to remove the noxious materials from this sub-flow.
Various constructions and arrangements of such mixing chambers in the bypass conduit are known. These known mixing chambers are all arranged at a relatively great distance from the point at which the bypass conduit branches off the connecting conduit between the rotary tubular kiln and the cyclone preheater.
In the operation of such known apparatus considerable problems have been frequently encountered. Due to the high temperature of the kiln waste gases the lining of the section of the bypass conduit which connects the conduit disposed between the rotary tubular kiln and the cyclone preheater to the mixing chamber is subject to considerable wear. Furthermore, since the sub-flow of the kiln gases passing through said bypass conduit frequently has a high dust content, and the noxious substances (such as alkalis and chlorides) have a strong tendency to form deposits in a critical temperature range, in these known constructions considerable depositing occurs in said section of the bypass conduit and in the mixing chamber, and this often leads to disturbances in operation and makes frequent cleaning necessary.
Accordingly, the object of the invention is to obviate these disadvantages and to construct an apparatus of this type in such a way that even with a high content of noxious substances in the kiln waste gases in the bypass conduit (in particular the section thereof leading to the mixing chamber) and in the mixing chamber itself, no troublesome deposits are formed.